The present invention relates generally to fiber optics and in particular to a fused fiber optical conduit displaying relatively blemish-free transmission qualities.
Fiber optics is based on the ability of smooth fibers of transparent materials such as glass to conduct light with high efficiency by means of multiple internal reflections and concomitantly the ability of each fiber in an array or bundle to conduct this light independently of its neighbors and without significant loss.
When light is incident at the interface between two dielectric media of different refractive index it will be totally reflected from the higher index surface for all angles of incidence greater than a specified critical angle related to the refractive indices of the two media. In actual practice, one finds that minute defects and contamination of the fiber surfaces interferes greatly with the total reflection phenomena by absorbing and scattering a fraction of the incident light thereby causing attenuation of the transmitted light and the appearance of blemishes on the viewing surface.
In addition to that caused by contamination and surface defects there is further loss of energy when the fiber cores come into intimate contact or within a distance of each other less than a wavelength of the transmitted light. This loss phenomena which is related to the tunnel effect of quantum mechanics is due to the finite penetration of electromagnetic waves into the rarer medium during conditions of total internal reflection. While the extent of this penetration is only in the order of a wavelength of light, should the adjacent fiber come within this distance then it will absorb the transmitted light and to that extent destroy the desired internal reflection.
The elimination of losses due to the aforementioned problems has, of course, been the object of much prior art. To a great extent the use of core cladding has obviated these defects and greatly improved the transmission qualities of the optical fibers. The use of cladding materials in intimate contact with the surface of the fiber cores provide a contaminant free core separated from its neighboring core by a distance equal to at least twice the thickness of the cladding material. In all instances this cladding material is kept within a range of thickness substantially greater than the wavelength of the light being transmitted and thusly prevents any cross talk or absorption of light by the neighboring fiber.
Despite the use of core claddings a significant number of blemishes still appear on the viewing surface of fiber optic arrays due to distortion of the core surfaces. When the core surface is disturbed it changes the angle of incidence of the light internal to the core thereby causing a portion of the transmitted light to have an angle of incidence less than the critical angle allowing its transmission through the wall of the fiber and its ultimate loss. Primarily these distortions of the core surface are the result of projections formed on the surface of neighboring cores during the fusion process. As the arrays of fiber cores are heated and drawn toward their ultimate desired dimensions, various random inclusions of foreign materials, etc. are vaporized causing the cladding to expand to a volume commensurate with the force developed. When this expanding volume or void forms at a point of contact with the neighboring fiber, the protrusion deforms the neighboring core surface. The effect of core surface deformations in producing blemishes or dark spots on the image surface of the fiber optic conduit is amply detailed and explained in United States patent application Ser. No. 51,784, filed July 2, 1970 entitled "Method of Producing An Improved Leachable Bundle of Optical Fibers and the Product Thereof", now U.S. Pat. No. 3,653,739 issued Apr. 4, 1972 of the same assignee as this application. The referenced application clearly demonstrates the effect of the bubbles or deformed surface of the fiber optic core on the transmission of light and its deleterious effect upon the transmission characteristics of the conduit.
Due to the high frequency of occurrence of blemishes in optical fiber conduits of practical length this problem seriously affects the transmission capacity of optical fibers in use today. The basis for this high defect rate is found mainly in the efforts made to increase the number of usable fibers within a given conduit. Obviously any increase in usable fibers within a conduit must add to the total information capacity or transmission capacity of the conduit itself. As the density of the fibers per unit of cross-sectional area of the conduit becomes higher, the contact surface of adjacent fibers increases. By this is meant that as the forces pressing radially inward become greater the fibers themselves tend to distort and adjacent fibers now contact each other along a rather large surface of the fiber as opposed to a point contact which may be maintained in a loosely packed or fused assembly. In addition free space within the bundle is eliminated or greatly diminished. Therefore as bubbles form there is no free volume to which they may travel and thus must produce pressures on the surface of the cores themselves thereby resulting in deformation of the core surface when forces associated with these bubbles becomes sufficiently high.
In the afore-referenced United States application, there is presented a method which obviates core distortion due to the formation of bubbles or voids in the glass cladding of the fibers. This improvement is attained by mounting on the circumferential periphery of the fibers a number of longitudinal rods fused to the exterior of the cladding. These rods are positioned with respect to each other in the bundle to maintain a certain separation between the fibers and to minimize the area of contact between the fibers, i.e. the contact area is limited to the surface area of the rod extending along the length of the fiber. This innovation accomplishes two central purposes. It maintains a certain amount of free space to which the bubbles may migrate and also minimizes the area of contact between the fibers thusly preventing capture of the bubbles at the fusion points between the fibers and their resultant impingement upon the core surfaces.
In view of the problems associated with blemishes in fiber optics and the devices employed in the prior art toward their elimination it becomes obvious that any further improvements directed to the elimination of core surface distortion are of particular interest and significance to the industry. The Applicants have directed their attention to a novel process and product eliminating the formation of blemishes on the image surface of optical fiber conduits. The Applicants' invention is further directed to the maximizing of fiber density to an extent which heretofore has not been compatible with the prior art methods for the elimination of core deformation due to the creation of bubbles at the fusion surfaces. It must be noted that the Applicants' invention overcomes the practical problems involved with the fusion assembly of prior art devices directed to this same problem and permits the utilization of existing techniques in the production of optical fiber conduits. Prior art production techniques and processes disclosed in U.S. Pat. No. 2,980,957 entitled "Method and Apparatus for Use in the Fabrication of Light-Conducting Devices", J. W. Hicks, Jr., Inventor and U.S. Pat. No. 3,037,241 entitled "Method and Apparatus for Manufacturing Light-Conducting Devices", W. P. Bazinet, Jr., et al, Inventor, may be used in the practice of the Applicants' invention to satisfactorily produce optical fiber conduits providing maximized light transmission with minimum occurrence of blemishes.
It is therefore an object of the Applicants' invention to provide a fiber optic conduit of improved light transmission quality. It is another object of the Applicants' invention to provide a process for producing relatively blemish-free fiber optical conduits. It is another object of the invention to provide a relatively blemish-free fiber optical conduit. It is yet another object of this invention to provide a relatively blemish-free flexible fiber optic conduit.